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// [js] 请实现一个flattenDeep函数，把多维数组扁平化

[].concat(...[1, 2, 3, [4, 5]]);  // [1, 2, 3, 4, 5]

function flatten(arr) {
  while (arr.some(item => Array.isArray(item))) {
    arr = [].concat(...arr)
  }
  return arr
}

ES10的flat方法
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